The present invention relates to a method for electrolytically refining copper at a high current density and more specifically to a method for electrolytically refining copper at a high current density, which never causes any passivation of the anode even when copper is subjected to electrolytic refining at a high current density, which never causes any adhesion of slime to the surface of the cathode, which never forms any nodule (particulate copper) and wrinkle-like unevenness on the cathode surface and which permits an increased yield of high purity copper without increasing the number of electrolytic cells.
In the electrolytic refining of copper, the electrolysis is in general carried out at a current density ranging from 200 to 350 A/m2. In this respect, if copper can electrolytically be refined while further increasing the current density, it would be possible to achieve an increased yield of refined copper without increasing the number of electrolytic cells. If the current density is increased, however, not only the anode undergoes passivation, but also wrinkle-like unevenness is generated on the cathode surface due to the enhancement of the spontaneous convection current generated in the space between the anode and the cathode. Moreover, another problem concerning the electrolytic refining of copper arises such that the slime suspended in the electrolyte adheres to the cathode surface to thus form nodules (particulate copper) on the cathode surface. In any case, the purity of the resulting electrolytically refined copper is reduced.
According to the conventional electrolyte circulation system used in the electrolytic refining of copper, the electrolyte is circulated at a velocity of not more than 0.2 m/s or in such a manner that the electrolyte is maintained at a state almost identical to the stationary state in order to prevent any suspension, in the electrolyte, of the slime adhered to the anode surface or settled on the bottom of the electrolytic cell. The use of such a low velocity of the electrolyte never permits the smooth supply of copper to the electrolytic cell and therefore, this makes any increase in the current density impossible.
In addition, there has been proposed a PR electrolysis method as a high current electrolytic refining process of copper. In this case, however, it is likewise necessary to circulate the electrolyte while maintaining such a condition that the electrolyte is substantially free of any suspension of the slime adhered to the anode surface or settled on the bottom of the electrolytic cell and in other words, the circulation rate of the electrolyte (the velocity of the electrolyte in the electrolytic cell) cannot be increased so much. For this reason, the current density can be increased only in limited extents. Furthermore, there has also been proposed a high current density electrolytic refining technique, which makes use of a channel-type electrolytic cell.
As will be clear from the foregoing description, it would be impossible to increase the current density in the electrolytic refining of copper simply by raising the circulation rate of the electrolyte (the velocity of the electrolyte in the electrolytic cell) without causing any problem, unless the slime suspended in the electrolyte is removed.
Accordingly, there has still been required for the development of a high current density electrolytic refining method of copper, which permits the solution of all of the problems concerning the passivation of the anode, any adhesion of slime onto the cathode surface, any formation of nodules (particulate copper) and/or wrinkle-like unevenness on the cathode surface and any reduction of the purity of the resulting copper and which permits the achievement of an increased yield of refined copper without increasing the number of electrolytic cells.
Accordingly, it is an object of the present invention to provide a high current density electrolytic refining method of copper, which never causes any passivation of the anode even when copper is electrolytically refined at a high current density, which never causes any adhesion of slime to the surface of the cathode, which never forms any nodule (particulate copper) and wrinkle-like unevenness on the cathode surface and which can ensure an increased yield of high purity copper without increasing the number of electrolytic cells.
The inventors of this invention have conducted various studies to achieve the foregoing object, have found that the foregoing problems associated with the conventional techniques can be eliminated by maintaining the electrolyte accommodated in an electrolytic cell at a temperature not less than a predetermined level, pouring the electrolyte into the electrolytic cell from the top thereof and discharging the same out of the bottom of the cell together with the slime and recirculating the electrolyte into the electrolytic cell after the removal of the slime, to thus allow the electrolyte to flow downward on the whole surface of the cathode and have thus completed the present invention.
Therefore, the method for electrolytically refining copper at a high current density according to the present invention relates to the electrolytic refining of copper, which makes use of crude copper as an anode, and is characterized in that the temperature of the electrolyte in an electrolytic cell is maintained at a level of not less than 55xc2x0 C.; that the electrolyte is continuously introduced into the electrolytic cell from the top of the cell, continuously discharged from the bottom of the cell along with slime, the slime is removed from the electrolyte discharged from the electrolytic cell and the electrolyte free of any slime is recirculated in the cell; and that the electrolyte is passed through the space between the electrodes at an average electrolyte velocity sufficient for allowing the electrolyte to flow on the whole surface of the cathode downward or in the direction opposite to that of the upward stream of the electrolyte generated on the cathode surface when the electrolysis is initiated while the electrolyte is in the stationary state.